This invention relates to a chemical vapor deposition process in integrated circuit fabrication and, more particularly, to a method of achieving high adhesion of CVD copper thin films on Tantalum Nitride (TaN) substrates.
The demand for progressively small, less expensive, and more powerful electronic products has fueled the need for smaller geometry integrated circuits (ICs) on large substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the interconnections between components and dielectric layers be as small as possible. Therefore, research continues into reducing the width of via interconnects and connecting lines. The conductivity of the interconnects is reduced as the area of the interconnecting surfaces is reduced, and the resulting increase in interconnect resistivity has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the IC. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected, or from the joining of conducts having highly different impedance characteristics.
There is a need for interconnects and vias to have both low resistivity, and the ability to withstand process environments of volatile ingredients. Aluminum and tungsten metals are often used in the production of integrated circuits for making interconnections or vias between electrically active areas. These metals are popular because they are easy to use in a production environment, unlike copper which requires special handling.
However, due to its unique properties, copper (Cu) appears to be a natural choice to replace aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. As a result, the same current can be carried through a copper line having nearly half the width of an aluminum line. The electromigration characteristics of copper are also much superior to those of aluminum. Aluminum is approximately ten times more susceptible than copper to degradation and breakage due to electromigration. As a result, a copper line, even one having a much smaller cross-section than an aluminum line, is better able to maintain electrical integrity. Accordingly, due to Copper""s low resistivity (1.7 xcexcxcexa9xc2x7cm) and high electromigration resistance, copper metal thin films are considered the ideal material for successful future use as metal interconnections in integrated circuit devices.
There have been problems associated with the use of copper, however, in integrated circuit processing. Copper interconnect lines generally are formed by depositing copper onto either dual damascene or single damascene trenches lined with a barrier metal, typically in the form of a metal nitride such as titanium nitride or tantalum nitride, followed by a chemical mechanical polishing (CMP) process. The deposition methods include plasma enhanced chemical vapor deposition (PECVD), metal organic chemical vapor deposition (MOCVD), and electrochemical deposition (ECD) processes. ECD requires a copper seed layer, which is either deposited by PECVD or by MOCVD. Copper deposited by PECVD exhibits poor step coverage. Accordingly, the process is not suitable for very narrow trench applications.
MOCVD appears to be the ideal process for the deposition of copper into sub-micron trenches and vias. MOCVD also appears to be the ideal process for providing copper seed for ECD trench/via filling. The problem, however, is that the state-of-the-art MOCVD process does not yield good adhesion between the copper and the barrier metal. The current solutions to this problem include using a flash PECVD process to provide a very thin seed for the seed CVD copper, or adding a small amount of silicon to the top of the barrier metal nitride compound. These solutions increase the process complexity and/or increase the barrier metal to copper contact resistance.
Modification of the copper precursor is another option to improve the adhesion of the copper thin film. In one example, CupraSelect Blend (Registered trademark of Schumacher company) was introduced to replace pure CupraSelect (Registered trademark of Schumacher company) by the addition of a very small amount of dihydride hexafluoroacetylacetone (H(hfac).2H2O). By using CupraSelect Blend and a copper precursor, the adhesion of copper thin films on a TiN substrate is improved. However, use of the CupraSelect Blend does not solve the adhesion problems associated with deposition of copper thin films on TaN substrates.
The method of the present invention provides a fabrication process for achieving high adhesion of CVD copper thin films on metal nitride substrates, and in particular, on Tantalum Nitride (TaN) substrates. The method comprises introducing a certain amount of water vapor to the initial copper thin film deposition stage and reducing the amount of fluorine in the interface of the copper and metal nitride substrate. These two process steps result in a copper thin film having improved adhesion to metal nitride substrates, including TaN substrates.
Accordingly, an object of the present invention is to provide a method of producing copper thin films having high adhesion to metal nitride substrates.
Another object of the present invention is to provide a method of producing copper thin films having high adhesion to TaN substrates.
Still another object of the present invention is to provide a method of introducing a certain amount of water vapor to an initial copper thin film deposition stage so as to produce a thin copper film having increased adhesion to TaN substrates.
Yet another object of the present invention is to provide a method of reducing the amount of fluorine in an interface so as to produce a thin copper film having increased adhesion to TaN substrates.